The present invention relates to a power assisted steering system for providing a hydraulic power assist to the driver's steering effort applied to the steering wheel of an automotive vehicle, and more particularly to a power assisted steering system of the type which includes a hydraulic reaction mechanism for controlling the hydraulic power assist in accordance with increase or decrease of the vehicle speed.
In a conventional power assisted steering system of this type, the hydraulic reaction mechanism is connected to a secondary hydraulic pump to be applied with a hydraulic pressure in proportion to the vehicle speed. In general, the secondary hydraulic pump is in the form of a hydraulic pump driven by a propeller shaft of the vehicle. Alternatively, the secondary hydraulic pump is in the form of a hydraulic pump associated with a primary hydraulic pump for the steering system to apply the hydraulic pressure to the reaction mechanism. However, the former type of hydraulic pump must be mounted on the propeller shaft and connected to the reaction mechanism by means of fluid conduits. This results in complicated construction of the steering system. In the latter type of hydraulic pump, it is necessary to avoid mutual interference between the hydraulic pressures respectively applied to the power cylinder and the reaction mechanism. This results in complicated construction of a control apparatus for the hydraulic pumps.